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CONTENTS
Volume 27, Number 4, November10 2007
 

Abstract
This paper presents the findings of an experimental study to evaluate retrofit methods which address particular weaknesses that are often found in reinforced concrete structures, especially older structures, namely the lack of the required flexural and shear reinforcement within the columns and the lack of the required shear reinforcement within the joints. Thus, the use of a high-strength fiber jacket for cases of post-earthquake and pre-earthquake retrofitting of columns and beam-column joints was investigated experimentally. In this paper, the effectiveness of the two jacket styles was also compared.

Key Words
evaluation and retrofit; buildings; structural response concrete; composite materials; cement grout.

Address
Aristotle University of Thessaloniki, School of Engineering, 541 24 Thessaloniki, Greece

Abstract
In this paper, an identification method of impact force is proposed for composite structures. In this method, the relation between force histories and strain responses is first formulated. The transfer matrix, which relates the strain responses of sensors and impact force information, is constructed from the finite element method (FEM). Based on this relation, an optimization model to minimize the difference between the measured strain responses and numerically evaluated strain responses is built up to obtain the impact force history. The identification of force history is performed by a modified least-squares method that imposes the penalty on the first-order derivative of the force history. Moreover, from the relation of strain responses and force history, an error vector indicating the force location is defined and used for the force location identification. The above theory has also been extended into the cases when using acceleration information instead of strain information. The validity of the present method has been verified through two experimental examples. The obtained results demonstrate that the present approach works very well, even when the internal damages in composites happen due to impact events. Moreover, this method can be used for the real-time health monitoring of composite structures.

Key Words
Ning Hu; Department of Engineering Mechanics, Chongqing University, Chongqing 400044, P.R. China
Department of Aerospace Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6-01,
Aoba-ku, Sendai 980-8579, Japan
Satoshi Matsumoto, Ryu Nishi and Hisao Fukunaga; Department of Aerospace Engineering, Tohoku University, Aramaki-Aza-Aoba 6-6-01, Aoba-ku, Sendai 980-8579, Japan

Address
impact force; identification; optimization model; PZT; accelerometer.

Abstract
Large changes in stiffness associated with cracking and yielding of reinforced concrete sections may be expected to occur during the dynamic response of reinforced concrete frames to earthquake ground shaking. These changes in stiffness in stories that experience cracking might be expected to cause relatively large peak interstory drift ratios. If so, accounting for such changes would add complexity to seismic design procedures. This study evaluates changes in an index parameter to establish whether this effect is significant. The index, known as the coefficient of distortion (COD), is defined as the ratio of peak interstory drift ratio and peak roof drift ratio. The sensitivity of the COD is evaluated statistically for five- and nine-story reinforced concrete frames having either uniform story heights or a tall first story. A suite of ten ground motion records was used; this suite was scaled to five intensity levels to cause varied degrees of damage to the concrete frame elements. Ground motion intensity was found to cause relatively small changes in mean CODs; the changes were most pronounced for changes in suite scale factor from 0.5 to 1 and from 1 to 4. While these changes were statistically significant in several cases, the magnitude of the change was sufficiently small that values of COD may be suggested for use in preliminary design that are independent of shaking intensity. Consequently, design limits on interstory drift ratio may be implemented by limiting the peak roof drift in preliminary design.

Key Words
reinforced concrete frames; interstory drift ratio; seismic response; seismic design.

Address
Mark Aschheim? and Edwin Maurer; Civil Engineering Department, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053, USA
JoAnn Browning; Civil, Enivironmental, and Architectural Engineering Department, University of Kansas, 2150 Learned Hall, 1530 W. 15th Street, Lawrence, Kansas 66045-7609, USA

Abstract
Reinforced concrete (RC) joint shear strength models are constructed using an experimental database in conjunction with a Bayesian parameter estimation method. The experimental database consists of RC beam-column connection test subassemblies that maintained proper confinement within the joint panel. All included test subassemblies were subjected to quasi-static cyclic lateral loading and eventually experienced joint shear failure (either in conjunction with or without yielding of beam reinforcement); subassemblies with out-of-plane members and/or eccentricity between the beam(s) and the column are not included in this study. Three types of joint shear strength models are developed. The first model considers all possible influence parameters on joint shear strength. The second model contains those parameters left after a step-wise process that systematically identifies and removes the least important parameters affecting RC joint shear strength. The third model simplifies the second model for convenient application in practical design. All three models are unbiased and show similar levels of scatter. Finally, the improved performance of the simplified model for design is identified by comparison with the current ACI 352R-02 RC joint shear strength model.

Key Words
reinforced concrete; Bayesian parameter estimation; beam-column connections; experimental database; joint shear strength model.

Address
Jaehong Kim; University of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering, 3139 Newmark Laboratory, 205 N. Mathews Ave., Urbana, Illinois, 61801, USA
James M. LaFave; University of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering, 3108 Newmark Laboratory, 205 N. Mathews Ave., Urbana, Illinois, 61801, USA
Junho Song; University of Illinois at Urbana-Champaign, Department of Civil and Environmental Engineering, 2207 Newmark Laboratory, 205 N. Mathews Ave., Urbana, Illinois, 61801, USA

Abstract
Analysis of transverse stresses at layer interfaces in a composite laminate has always been a challenging task. Composite structures possess highly irregular material properties at layer interfaces, which cause high shear stresses. Classical Plate Theory and First Order Shear Deformation Theory (FSDT) use post computing to calculate transverse stresses. This paper presents Reissner Mixed Variational Theorem (RMVT) based finite element model to carry out layer-wise analysis of composite laminates. Selective integration scheme has been used. The formulation has been validated by solving numerical examples and comparing the results with those published in the literature.

Key Words
laminated composites; layer-wise model; transverse stress; mixed formulation; Reissner Mixed Variational Theorem; Cz0 requirements.

Address
S. S. Phoenix, M. Sharma and S. K. Satsangi; Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology, Kharagpur-721 302, West Bengal, India

Abstract
The main purpose of this paper is to investigate the ultimate behavior of steel cable-stayed bridges with design variables and compare the validity and applicability of computational methods for evaluating ultimate load capacity of cable-stayed bridges. The methods considered in this paper are elastic buckling analysis, inelastic buckling analysis and nonlinear elasto-plastic analysis. Elastic buckling analysis uses a numerical eigenvalue calculation without considering geometric nonlinearities of cablestayed bridges and the inelastic material behavior of main components. Inelastic buckling analysis uses an iterative eigenvalue calculation to consider inelastic material behavior, but cannot consider geometric nonlinearities of cable-stayed bridges. The tangent modulus concept with the column strength curve prescribed in AASHTO LRFD is used to consider inelastic buckling behavior. Detailed procedures of inelastic buckling analysis are presented and corresponding computer codes were developed. In contrast, nonlinear elasto-plastic analysis uses an incremental-iterative method and can consider both geometric nonlinearities and inelastic material behavior of a cable-stayed bridge. Proprietary software ABAQUS are used and user-subroutines are newly written to update equivalent modulus of cables to consider geometric nonlinearity due to cable sags at each increment step. Ultimate load capacities with the three analyses are evaluated for numerical models of cable-stayed bridges that have center spans of 600 m, 900 m and 1200
m with different girder depths and live load cases. The results show that inelastic buckling analysis is an effective approximation method, as a simple and fast alternative, to obtain ultimate load capacity of long span cable-stayed bridges, whereas elastic buckling analysis greatly overestimates the overall stability of cable-stayed bridges.

Key Words
elastic buckling analysis; inelastic buckling analysis; nonlinear elasto-plastic analysis; ultimate load capacity; cable-stayed bridge.

Address
D. H. Choi, H. Yoo, J. I. Shin and S. I. Park; Department of Civil Engineering, Hanyang University, 17 Haengdang-dong, Seoungdong-gu, Seoul 133-791, Korea
K. Nogami; Department of Civil Engineering, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji-shi,Tokyo 192-0397, Japan

Abstract
This study presents a model to better understand the shear behavior of reinforced concrete walls subjected to lateral load. The scope of the study is limited to squat walls with height to length ratios not exceeding two, deformed in a double-curvature shape. This study is based on limited knowledge of the shear behavior of low-rise shear walls subjected to double-curvature bending. In this study, the wall ultimate strength is defined as the smaller of flexural and shear strengths. The flexural strength is calculated using a strength-of-material analysis, and the shear strength is predicted according to the softened strut-and-tie model. The corresponding lateral deflection of the walls is estimated by superposition of its flexibility sources of bending, shear and slip. The calculated results of the proposed procedure correlate reasonably well with previously reported experimental results.

Key Words
reinforced concrete; double-curvature; squat wall; shear wall; strength; deflection; strut; tie.

Address
Ika Bali; Department of Civil Engineering, Universitas Kristen Indonesia, Jalan Mayjen Sutoyo, Cawang, 13630, Indonesia
Shyh-Jiann Hwang; Department of Civil Engineering, National Taiwan University, Taipei, 10617, Taiwan

Abstract
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Key Words
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Address
Muhammad Ashiqur Rahman; Department of Mechanical Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh
Junji Tani; Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan

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